def up_level(self):
	if self.level < 3:
		self.level += 1
	try:
		self.tank = pygame.image.load(self.tanks[self.level]).convert_alpha()
	except:
		self.tank = pygame.image.load(self.tanks[-1]).convert_alpha()

# 等级降低
def down_level(self):
	if self.level > 0:
		self.level -= 1
	self.tank = pygame.image.load(self.tanks[self.level]).convert_alpha()